Unless you want to take a selfie every five minutes, showing your actual facial expression on Tumblr is not very workable -- but hey, that's what emoticons are for. Unlike instant messaging programs and other social media you might've used before, Tumblr doesn't automatically turn text emoticons into animated images; if you want to add an animated emoticon to a post, you can insert it manually, provided it's in the animated GIF format.
In Text Posts
When composing text posts, you can upload images, including animated emoticons, directly to Tumblr from within the edit screen for the post. Place the text cursor where you want the emoticon to appear, click the "Upload Image" button, browse to the location of the emoticon on your hard drive and double-click the file. The emoticon will upload to Tumblr and appear in your post. If you want, you can also insert images that are already hosted somewhere on the Internet, but you'll need to click the "HTML" button and insert the raw HTML code for the image yourself (see Resources).
In Other Post Types
When composing posts that aren't of the text type, you can only add emoticons that have already been uploaded to the Internet. Click the "Insert/Edit Image" button, paste the URL to the emoticon in the "Image URL" field and then click "Insert." If the emoticon is on your hard drive, you'll first need to upload it to a hosting service such as TinyPic, Photobucket, ImageShack or Imgur (see Resources).